Improvement of crop plants for a variety of traits, including disease and pest resistance, and grain quality improvements such as oil, starch or protein composition, can be achieved by introducing new or modified genes (transgenes) into the plant genome. Transcriptional activation of genes, including transgenes, is in general controlled by the promoter through a complex set of protein/DNA and protein/protein interactions. Promoters can impart patterns of expression that are either constitutive or limited to specific tissues or times during development.
Transcriptional activation is primarily mediated through transcription factors that interact with enhancer and promoter elements. Binding of transcription factors to such DNA elements constitutes a crucial step in transcriptional initiation. Each transcription factor binds to its specific binding sequence in a promoter and activates expression of the linked coding region through interactions with coactivators and/or proteins that are a part of the transcription complex.
Several plant genes have been identified that appear to encode transcription factors structurally related to the cMyb protooncogene family of mammals. Central to the similarities shared by these proteins is the Myb repeat DNA-binding domain containing conserved tryptophan residues at certain positions, and a helix-turn-helix-related domain. Generally, Myb-related proteins from plants contain two of these repeats, R2 and R3 (Kranz et al. (1998) Plant J 16:263-276), though proteins having only one repeat have been identified (e.g., Feldbrugge et al. (1997) Plant J 11:1079-1093). These Myb-related genes appear to encode a large family of plant transcription factors that are involved in a diversity of gene regulation. For example, plant Myb-related genes have been shown to regulate anthrocyanin biosynthesis in maize and phenylpropanoid metabolism, disease resistance (WO9813486-A1), expression of gibberellin-regulated genes (WO9700961-A1), expression of stress-related genes (WO9916878-A1), active carbohydrate secretion and flavonol metabolism in antirrhinum flowers (Jackson et al. (1992) Plant Cell 3(2):115-125). The first plant transcription activator gene described at the molecular level was the maize c1 gene which encodes a Myb protein (Paz-Ares et al. (1987) EMBO J. 16:3553-3558) involved in regulating anthocyanin biosynthesis by trans-activating genes such as c2, A1 and Bz1 which encode enzymes involved in the pathway.
There is a great deal of interest in identifying the genes that encode proteins involved in transcriptional regulation in plants. These genes may be used in plant cells to control gene expression constitutively, in specific tissues or at various times during development. Accordingly, the availability of nucleic acid sequences encoding all or a portion of a Myb-related transcription factor would facilitate studies to better understand gene regulation in plants and provide genetic tools to enhance or otherwise alter the expression of genes controlled by Myb-related transcription factors.